Radio communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such communication networks support communications for multiple communication equipments (CEs) by sharing the available network resources. One example of such a network is the Universal Mobile Telecommunications System (UMTS), a third generation (3G) mobile phone technology standardized by the 3rd Generation Partnership Project (3GPP). UMTS includes a definition for a Radio Access Network (RAN), referred to as UMTS Terrestrial Radio Access Network (UTRAN). The UMTS, which is the successor to Global System for Mobile Communications (GSM) technologies, supports various air interface standards, such as Wideband-Code Division Multiple Access (W-CDMA), Time Division-Code Division Multiple Access (TD-CDMA), and Time Division-Synchronous Code Division Multiple Access (TD-SCDMA). The UMTS also supports enhanced 3G data communications protocols, such as High Speed Packet Access (HSPA), which provides higher data transfer speeds and capacity to associated UMTS networks. As the demand for mobile broadband access continues to increase, research and development continue to advance the UMTS technologies not only to meet the growing demand for mobile broadband access, but to advance and enhance the user experience with mobile communications. For example, third-generation UMTS based on W-CDMA has been deployed in many places the world. To ensure that this system remains competitive in the future, 3GPP began a project to define the long-term evolution of UMTS cellular technology. The specifications related to this effort are formally known as Evolved UMTS Terrestrial Radio Access (E-UTRA) and Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), but are more commonly referred to by the name Long Term Evolution (LTE). More detailed descriptions of radio communication networks and systems can be found in literature, such as in Technical Specifications published by, e.g., the 3GPP. The core network (CN) of the evolved network architecture is sometimes referred to as Evolved Packet Core (EPC) and when referring to a complete cellular system, including both radio access network and core network, as well as other possible entities, such as service related entities, the term Evolved Packet System (EPS) can be used.
Time Division Duplexing (TDD)
In a TDD system, UL and downlink (DL) transmissions occur on the same frequency band but at different time instances. A given subframe may only be allocated for transmission in either UL or DL.
FIG. 1 schematically illustrates a system 10. The system 10 comprises a CE 11 (e.g. a UE) communicating with a network node 12 (e.g. a base station such as eNB) in both UL and DL.
In a static TDD system, a fixed pattern (i.e. TDD configuration) specifies which subframes are designated for UL and DL, respectively.
In a fully dynamic TDD system, there is no fixed pattern that specifies which subframes are designated for UL and DL, respectively. Instead, which subframes that are to be used for UL and DL transmissions is generally decided continuously or “one-the-fly” by a scheduler of a network node (e.g. a base station) depending on the UL/DL traffic pattern. Certain restrictions on what the subframes are used for do apply, however. Some subframes are fixed to be DL subframes to allow transmission of, e.g., DL control information and channel state information-reference symbols (CSI-RS). Other subframes are fixed for UL transmissions of UL control information and/or random-access signaling.
There also exist semi-static TDD systems. In a semi-static TDD system, such as LTE Release 12, it is possible to switch between TDD configurations. This may e.g. be done in a slow time frame using higher-layer signaling. Typically, but not necessarily, the switches may be performed every frame, i.e. every 10 ms.
Multi-Subframe Scheduling
LTE DL was designed so that one scheduling message schedules one data transmission in UL or reception in DL. Multi-subframe scheduling implies that a scheduling message schedules multiple subframes. An example of a multi-subframe scheduling mechanism is described in the United States Patent Application Publication No. US2014/0301299A1. According to the disclosure of US2014/0301299A1, if mobile devices can only be configured for either multi-subframe scheduling or normal signal sub-subframe scheduling, mobile devices configured for only multi-subframe scheduling would not be able to be scheduled for a high quantity of subframes in advance. US2014/0301299A1 suggests solving this potential problem by adding one more flag bit in a Downlink Control Information (DCI) message to indicate whether the single-subframe scheduling scheme or the multi-subframe scheduling scheme is used for this DCI message. The main idea of US2014/0301299A1 is to introduce a multi-subframe scheduling activation pattern over RRC (abbreviation for Radio Resource Control). This bitmap pattern points out special subframes for which the UE shall assume that the received DCI message is valid for N>1 subframes instead of only this subframe. This behavior is also on/off-controlled by a flag bit in the RRC. So if the flag is on (i.e., enabled) and the bitmap over RRC is 0100010010 and N=2, this would that scheduling in subframe 2, 6 and 9 is also valid for subframes 3, 7, 10.
In fully dynamic TDD and in a scenario with a high demand of UL-subframes there may be a single DL subframe followed by n consecutive UL subframes, where n may be on the order of several tens. In this scenario, all n UL subframes must be scheduled by the base station and granted to a communication equipment, such as a user equipment (UE), in said single DL subframe. This is another example of multi-subframe scheduling. The resource allocation granted to the UE is then applied to all subsequent UL subframes and is valid until a new UL grant is received by the communication equipment.
In DL, multi-subframe scheduling is not as much of an issue as it is for UL. If a given subframe is used for DL there can also be a DL grant transmitted on the EPDCCH, which details the resource allocation of the PDSCH transmission. A potential advantage of using multi-subframe scheduling on DL would be that fewer grants need to be transmitted and space can be saved on the EPDCCH.
In UL, the advantages of multi-subframe scheduling are even more accentuated. When scheduling UL transmissions when multi-subframe scheduling cannot be used, a new UL grant has to be transmitted on a DL control channel, e.g., the EPDCCH. This forces a DL transmission using a DL subframe, which in an UL-heavy scenario may be very wasteful since this subframe cannot convey any UL data.
Beamforming
In evolved LTE and future 5G systems the technology of beamforming is envisioned to become a key component. Through the use of array antennas the transmitted and/or received signal can be steered in specific directions. Beamforming comes in two distinct variants with some key differences. The most advanced, and also most demanding when it comes to processing requirements, is typically referred to as “digital beamforming”. This enables individual PRBs to be steered in a desired direction without dependencies to other PRBs and their beam directions. Hence, digital beamforming makes CE-specific beam-steering possible. There also exists a beamforming technique known as “analog beamforming”, which only enables the entire bandwidth, i.e., all PRBs, to be steered in the same direction. Hence, no CE-specific beam-steering is possible. Typically, it is possible to transmit/receive a small number of different analog beams in a given subframe. However, the number of beams available is normally quite limited; say in the order of eight.